Seebeck and Nernst effects in the mixed state of the two-band organic superconductors κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br

The in-plane longitudinal (Seebeck) effect and the transverse (Nernst) effect in magnetic field of the anisotropic superconductors κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br have been studied in the vicinity of the superconducting transition. Anomalous sign changes of the Seebeck as well as of the Nernst voltage were found for the temperature gradient along the direction in which the electronic transport is dominated by the electron-like carriers. Both effects are attributed to the flux motion under the temperature gradient. The experimental results are interpreted taking into account the anisotropic two-band nature of the conducting system in these compounds and assuming a predominant role of one of the bands (namely, the electron-like one) in the superconducting pairing.